<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <!-- 编写函数 is_string(arg) 判断传入的内容是否为字符串 -->
</head>
<body>
  <script>
    function is_string(arg){
      if(typeof(arg) === 'string'){
        alert(true)
      }else{
        alert(false)
      }
    }
    is_string('34')
    
  </script>
</body>
</html>